It is known, for example, in financial institutions to use such bags to transfer sums of money between departments- A variety of different tamper evident bag designs are available, including that disclosed in EP-A-0 396 428, as illustrated in FIG. 1 of the accompanying drawings and described in detail below.
A problem which arises with such bags is that the adhesive used on the tamper evident closure portion, which is used to close the bag and seal the access opening, fails to work adequately, if wetted, and the bag can be opened without visibly affecting the tamper evident closure portion.
The surface tension of all presently available substances which are used in pressure sensitive tamper evident closures, to seal such bags, is approximately 32 Dynes cm.sup.1. Therefore, the wetting of the closure with water, which has a surface tension of approximately 70 Dynes cm.sup.1 and a low viscosity, is not an insurmountable problem. This is because the majority of the water used will be ejected by the action of pressing the closure to the bag. Also, due to its higher surface tension the water which is left will form droplets instead of smearing along the length of the closure, and will only cause localised failure of the seal. However, saliva, which has a surface tension of approximately 30-32 Dynes cm.sup.1 and a much higher viscosity than water, will not be ejected by the action of the closure and will smear over the latter thus preventing adequate sealing of the bag.
Therefore, presently available tamper evident bags have the disadvantage that someone can pretend to seal the bag, but lick it beforehand, thus preventing sealing of the bag and enabling unauthorised access to the interior thereof.
It is an object of the present invention to address the aforementioned disadvantage of presently available tamper evident bags.
One apparent solution to this problem is to form a bag from a material with a reduced effective surface tension such that saliva will not smear on the bag and will not cause failure of the seal along its entire length. However, as discussed above the lowest possible surface tension of the material used on the closure to seal the bag is approximately 32 Dynes cm.sup.1. Therefore, the closure will act as a medium to smear the saliva, when pressed against the bag, and will thus prevent complete closure and sealing of the bag.